1. Field of the Invention
The present invention relates to an optical switching element and to a device, a photoaddressable display medium, and a display device each using the optical switching element.
2. Description of the Related Art
In recent years, photoaddressable spatial modulator devices have been developed in which a photoconductive switching element and a display element are combined. Such devices are in actual use as a light bulb of projectors or the like and have been investigated for potential use in the field of optical information processing as disclosed in “Ekisho Kukan Henchoki to Joho-Shori” in Japanese (Liquid Crystal Spatial Modulator and Information Processing), Ekisho (Liquid Crystal), Vol. 2, No. 1, 1998, pp 3-18.
In the photoaddressable spatial modulator device, the display element is driven to form display images by applying a certain voltage to the element, allowing the photoconductive switching element to change in impedance depending on the amount of received light, and controlling the voltage applied to the display element.
In particular, there is a medium, in which a display element having a memory effect and a photoconductive switching element are layered, and the medium is addressable (writable) by applying voltage and optical image input. Such a medium has attracted attention as an electronic paper medium, which can be carried without carrying a writing device with it.
Examples of the display element that has been investigated for the photoaddressable medium include liquid crystal display elements such as those using a cholesteric liquid crystal or a ferroelectric liquid crystal, an electrophoretic element, an electrorotation element, a toner electrotransfer element, and an encapsulated element of these.
Examples of the optical switching element that can control voltage or current depending on the amount of received light include an amorphous silicon element used in the field of electrophotography, an OPC element that has a function separation type double-layer structure using an organic photoconductor, and an OPC element investigated by the inventors that has a structure including a carrier transporting layer (CTL) and carrier generating layers (CGLs) provided on both sides of the CTL (hereinafter referred to as a dual CGL structure). In particular, the OPC elements can advantageously be formed in a flexible substrate such as a PET film since they do not require a heat treatment at high temperature. Moreover, they can be manufactured at low cost since they do not require a vacuum process. The dual CGL structure is particularly advantageous, because it can be driven by alternating current and because even a liquid crystal element used as the display element can resist generating image-sticking phenomenon, which would otherwise be caused by ion transfer under the bias component included in the applied voltage. The carrier used for driving may be positive or negative.
FIG. 15 schematically shows a photoaddressable electronic paper (a photoaddressable display medium) utilizing the optical switching element.
The display medium 100 comprises a pair of substrates 102, electrodes 101 each formed on the surface of each substrate, and a display layer 104 (a display element) and an optical switching layer 106 (an optical switching element) both connected to the electrodes, respectively, and layered and sandwiched between the substrates 102. While a pulse voltage is applied to the display medium 100 from writing pulse applying means 110, light is applied from an optical writing unit 108 based on image information, allowing the display medium 100 to display images.
Use of cholesteric liquid crystal in the photoaddressable system has been proposed for colorization of the photoaddressable electronic paper (photoaddressable display medium). Among color electronic paper technologies, this system can display additive color mixtures by lapping RGB without using color filters two-dimensionally to exhibit colors such as RGB. This system therefore has got into the limelight as a promising technology. For practical use of such a color display system, for example, Japanese Patent Application Laid-Open (JP-A) No. 2000-111942 proposes a color display medium using a cholesteric liquid crystal display element.
In the color display medium using the cholesteric liquid crystal display element as disclosed in JP-A No. 2000-111942, however, the conventional optical switching element has had a problem that its switching performance is insufficient in low electric field. As shown by the dependence of the resistance component on AC voltage in FIG. 16, the conventional optical switching element can offer only significantly degraded on/off switching performance in an electric field of 10 V/μm or lower.
In the photoaddressable electronic paper using cholesteric liquid crystal capsules, a voltage of about 250 V0p is generally applied to the medium to form display images from applied optical images, as disclosed in SID INTERNATIONAL SYMPOSIUM DIGEST of TECHNICAL PAPAERS, June 2001, “A Novel Photoaddressable Electronic Paper Utilizing Cholesteric LC Microcapsules and Organic Photoconductor.” As recited in this report, the threshold values of the display layer and integrated medium should be about 200 V and about 250 V, respectively, and about 50 V should be applied to the switching element. The thickness of the optical switching layer in the medium is about 3 μm, and therefore, an electric field of about 13 V/μm should be applied to the switching element. The optical switching element can have a brightness ratio of about 50 in an applied electric field of 10 V/μm or higher, and thus, the switching on/off margin is sufficient.
In the color display medium using the cholesteric liquid crystal display element such as disclosed in JP-A No. 2000-111942, an electric field of 10 V/μm or higher is generally applied to the optical switching layer (the photoconductive layer) when the voltage is applied to achieve Vfh 10 or Vfh 90, but an electric field of 10 V/μm or lower, generally about 3 V/μm is applied to the optical switching layer (the photoconductive layer) when the voltage is applied to achieve Vpf 10 or Vpf 90. In a color display, brightness switching at Vpf 10 or Vpf 90 is required of the optical switching layer (the photoconductive layer) (see FIG. 16 of JP-A No. 2000-111942). As shown in FIG. 16, however, the performance of the optical switching generally used in SID is poor in an electric field of 10 V/μm or lower, and the brightness on/off ratio of the resistance component is apparently too small. Thus, color switching cannot be performed sufficiently by the conventional color display medium.
The problem is that the bright time can have no assured photoconductivity or no reduced resistance in low electric field applied, although the dark time can have high resistance as well as high electric field. If it should be driven, the resistance value per unit of film thickness has to be reduced, for example, to about 2 MΩ or less.
Even in a monochrome medium, when a cholesteric liquid crystal display element is used, a system may be used in that a high electric field is once applied to form a highly reflective planar domain at the front face, and then the planar domain is selectively made a low reflective focal conic. In such a system, the optical switching element also has to be driven in low electric field, because the electric field is used to change the phase from the planar phase to the focal conic phase.
In a display medium using the cholesteric liquid crystal display element, therefore, the optical switching element has to be driven over a wide electric field range from high to low. Thus, there has been a demand for improvement in this respect.
In the display medium using an electrochromic or electro liquid powder display element, the threshold itself is low, and therefore, low electric field is needed for driving. Though the optical switching element does not have to be driven over a wide electric field range from high to low in such a case, the optical switching element must be driven in low electric field, and therefore, there has been a demand for improvement in this respect.